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潮汐对闽江下游感潮河段表层水体4种盐基离子浓度的影响

The influence of tide on the concentrations of four base cations in surface water of tidal reach in the lower reaches of Minjiang River

  • 摘要: 研究河流感潮河段涨、落潮过程中水体盐基离子浓度分布特征及其影响因素对认识入海河流生态环境具有重要意义。以闽江下游感潮河段(长117 km,分为上河段、中河段和下河段)为研究对象,在巨潮日随涨潮和落潮过程,采集11个采样点的表层水体,分析水体中K+、Ca2+、Na+和Mg2+盐基离子浓度及其与环境因子间的关系。研究结果表明,水体中K+、Ca2+、Na+和Mg2+离子质量浓度分别为2.28~448.18 mg/L、7.34~257.89 mg/L、4.54~7 898.50 mg/L和1.39~892.44 mg/L,在感潮河段的上河段和中河段变化较小,在下河段显著上升。沿感潮河段从上河段至下河段,水体中K+和Ca2+离子浓度占比显著降低,Na+离子浓度占比明显升高,而Mg2+离子浓度占比较为稳定。相对于涨潮,落潮过程一定程度降低了K+、Ca2+、Na+和Mg2+离子浓度,尤其在下河段。盐基离子浓度与pH、盐度和悬浮物浓度均显著正相关。综合分析表明,潮汐主要通过改变淡、盐水混合,影响盐基离子浓度及其分布,对认识潮汐作用下感潮河段盐基离子迁移特征及陆海物质循环具有科学意义。

     

    Abstract: Rivers serve as a link between the land and the ocean, transporting a large amount of terrigenous substances from the land to the sea. The instability of the concentration of base cations in rivers has a significant impact on the water environment of the river basin. Investigating distribution characteristics of base cations and their influencing factors during tidal cycles in the tidal reaches of rivers is of great significance for understanding the ecological environment of rivers flowing into the sea. The 117-km tidal reach of the lower Minjiang River (divided into upper, middle, and lower segments) was selected as the study area, and the surface water at 11 sampling points was collected during the flood tide and ebb tide processes on a spring tide day. The concentrations of base cations such as K+, Ca2+, Na+ and Mg2+ in the water and their relationships with environmental factors were further analyzed. The results show that the mass concentrations of K+, Ca2+, Na+ and Mg2+ cations in the water are 2.28-448.18 mg/L, 7.34-257.89 mg/L, 4.54-7 898.50 mg/L and 1.39-892.44 mg/L, respectively. There are relatively small changes in measured base cations in the upper and middle reaches of the tidal reach, but the lower reach exhibits a significant increase. In the upper and middle reaches of the tidal reach, the proportions of base cations concentrations such as K+, Ca2+, Na+ and Mg2+ in the water are 12.26%-15.31%, 32.66%-47.63%, 29.03%-45.46% and 8.61%-9.63%, respectively. In the lower reach, the proportions of K+ and Ca2+ cations in the water are significantly decreased, and the proportion of Na+ cations is dramatically increased, while the proportion of Mg2+ cations is relatively stable. Compared with the flood tide, the ebb tide process decreases the concentrations of K+, Ca2+, Na+ and Mg2+ cations to a certain extent. Especially in the lower reach, the decline ratios are about 25.19%-69.48%, 19.18%-53.75%, 22.61%-87.13% and 21.47%-81.67%, respectively. During the flood tide and ebb tide processes, the concentration of base cations is significantly positively correlated with pH, salinity and the concentration of suspended substances. During the ebb tide process, the concentration of base cations also has an obvious positive correlation with the water temperature. Comprehensive analysis further shows that tides mainly affect the concentrations and distribution of base cations by changing the mixing of fresh water and salt water, and the upstream movement of seawater caused by the flood tide has a more obvious impact on this process. These results not only reflect the hydrochemical characteristics of the river basin under the action of natural processes and human activities, but also help to understand the complex material migration process between the land and the sea. Nevertheless, this study only sampled on a spring tide day with strong tidal effects, lacking a comprehensive exploration of the concentration and distribution of base ions on neap tides. Overall, our results provide a scientific basis for the ecological protection and sustainable development of the river basin.

     

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